Saccharides play important roles in biological processes. During the last decade, much effort has been directed toward developing fluorescent chemosensors for detecting saccharides. One issue to the design of such sensors is the availability of fluorescent reporter moieties that respond to the saccharide recognition event with significant fluorescence intensity changes. Another issue in the synthesis of such sensors is the low water solubility of most fluorescent reporter compounds that are used for making fluorescent sensors.
The use of fluorescent sensors for cell-surface polysaccharides for in vivo, in vitro, and other biological applications is particularly attractive. The additional requirements for a sensor functional in vivo, in vitro, and other biological applications include: (1) reasonable water solubility; (2) optimal fluorescence intensity changes at physiological pH; and (3) chemical and photochemical stability.
Boronic acids have been known for decades to bind saccharides via covalent interaction. The most common interactions are with cis-1,2- or 1,3-diols of saccharides to form five- or six-membered rings, respectively. During the last decade, progress has been made in the construction of boronic acid-based sensors for carbohydrates. Different mechanisms have been used to induce spectroscopic changes upon binding of the boronic acid moiety with a saccharide. Among the most important discoveries is an anthracene-based fluorescent reporter system developed by Shinkai and co-workers. See T. D. James, K. R. A. S. Sandanayake, and S. Shinkai, Chem. Commun. 1994, 477-78; K. R. A. S. Sandanayake and S. Shinkai, J. Chem. Soc. Chem. Commun. 1994, 1083-84. This anthracene-based system has been widely used because of its fairly large fluorescence intensity increase upon ester formation due to the switching of a photoinduced electron transfer (PET) process.
The anthracene-based fluorescent reporter, however, has many undesirable properties, such as low water solubility and poor photochemical stability. For such sensors to be useful in a biological system and/or testing, it is essential that they function under near physiological conditions. One such fluorescent saccharide sensor, 8-quinolineboronic acid (8-QBA), responds to the binding of a carbohydrate with unprecedented large increases in fluorescence intensity at physiological pH in aqueous solution. See W. Yang, J. Yan, G. Springsteen, and B. Wang, Bioorg. Med. Chem. Lett. 2003, 13, 1019-1022.
Described herein are water soluble, boronic acid fluorescent reporter compounds that can be used as sensors for a variety of small molecule analytes and macromolecules such as, for example, carbohydrates and glycosylated macromolecules including glycolipids, glycoproteins. The compounds described herein are sufficiently water soluble and exhibit very large fluorescence intensity changes upon binding to a macromolcule. The use of these compounds as potential fluorescent sensors for small molecule analytes and macromolecules, such as saccharides, takes advantage of the hybridization state change of the boron upon binding to these molecules at physiological pH, which causes a change in the spectroscopic properties of the boron-containing compounds.